I. Field
The following description relates generally to wireless communications, and more particularly to providing a mechanism for switching between OFDM-MIMO and LFDM-SIMO techniques.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as, for example, voice, data, and so on. Typical wireless communication systems may be multiple-access systems capable of supporting communication with multiple users by sharing available system resources (e.g. bandwidth, transmit power, . . . ). Examples of such multiple-access systems may include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiplexing (OFDM), localized frequency division multiplexing (LFDM), orthogonal frequency division multiple access (OFDMA) systems, and the like.
Generally, wireless multiple-access communication systems may simultaneously support communication for multiple mobile devices. Each mobile device may communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to a communication link from base stations to mobile devices, and the reverse link (or uplink) refers to a communication link from mobile devices to base stations. Further, communications between mobile devices and base stations may be established via single-input single-output (SISO) systems, multiple-input single-output (MISO) systems, multiple-input multiple-output (MIMO) systems, single-input multiple output (SIMO), and so forth.
MIMO systems commonly employ multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which may be referred to as spatial channels, where NS≦{NT, NR}. Each of the NS independent channels corresponds to a dimension. Moreover, MIMO systems may provide improved performance (e.g., increased spectral efficiency, higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and received antennas are utilized.
MIMO systems may support various duplexing techniques to divide forward and reverse link communications over a common physical medium. For instance, frequency division duplex (FDD) systems may utilize disparate frequency regions for forward and reverse link communications. Further, in time division duplex (TDD) systems, forward and reverse link communications may employ a common frequency region.
SIMO systems commonly employs a single transmit antenna and plurality of receive antenna. SIMO systems can be used to perform beam-forming by combining antenna signals to point in a specific direction. Further, receive combining diversity, where antenna signals are combined to optimally adapt to local channel conditions, can be achieved using SIMO systems. One well-known technique is Maximum-Ratio-Combining (MRC), in which antenna signals are weighted, phase-aligned, and added in such a way as to maximize the signal-to-noise (SNR) ratio.
The OFDM system has higher peak to average ratio (PAR) than single carrier wave forms. This is true at all SNR ranges, however, the overall link efficiency between OFDM and LFDM techniques depends on the operating SNR as well as MIMO capability of the users. The PAR has more dominant impact on the power limited users, (e.g. the users with low operating SNR at the cell edges). For power limited users, the transmission data rate is limited by the power amplifier (PA) headroom. In order to operate in the linear region of the PA, one has to back off more in the OFDM case due to the increased PAR. The link efficiency of OFDM is small compared to OFDM for the low SNR case. Overall, the link loss due to the PAR back off outweighs the link efficiency achieved by OFDM technique, therefore, it is more advantageous to use LFDM. In fact some other low PAR system, such as interleaved frequency domain multiplexing (IFDM) system, will have the same tradeoff such as LFDM vs. OFDM as well. For the high SNR users, on the other hand, the performance advantage of OFDM compared to LFDM is significant. This is especially true for the high end MIMO users close to the base station.